Avoid having two PKs in a partition

Avoid having two PKs in a partition

If a table containing a primary key is attach as partition to a
partitioned table which has a primary key with a different definition,
we would happily create a second one in the new partition.  Oops.  It
turns out that this is because an error check in DefineIndex is executed
only if you tell it that it's being run by ALTER TABLE, and the original
code here wasn't.  Change it so that it does.

Added a couple of test cases for this, also.  A previously working test
started to fail in a different way than before patch because the new
check is called earlier; change the PK to plain UNIQUE so that the new
behavior isn't invoked, so that the test continues to verify what we
want it to verify.

Reported by: Noriyoshi Shinoda
Discussion: https://postgr.es/m/DF4PR8401MB102060EC2615EC9227CC73F7EEDF0@DF4PR8401MB1020.NAMPRD84.PROD.OUTLOOK.COM